Many high-sensitivity microelectromechanical systems (MEMS) such as microgyroscopes and some pressure sensors need to operate in hermetically sealed vacuum electronic packages to realize their full performance characteristics. Several hermetic sealing technologies for vacuum packaging of a carrier exist. Domes are currently fixed into carriers and then sealed in place by eutectic bonding, adhesive bonding, or silicon-glass anodic bonding. Each of the sealing methods runs a risk of compromising the vacuum within the package due to outgassing.
Outgassing is the slow release of a gas that was trapped, frozen, absorbed or adsorbed in some material. Outgassing can occur due to desorption, seepage from cracks or internal volumes and gaseous products of slow chemical reactions. Outgassing is a challenge to creating and maintaining clean high-vacuum environments. For example, outgassed products can condense onto optical elements, thermal radiators, or solar cells and obscure them. Materials not normally considered absorbent can release enough light-weight molecules to interfere with industrial or scientific vacuum processes. Even metals and glasses can release gases from cracks or impurities, but moisture, sealants, lubricants, and adhesives are the most common cause.
Gold and Silicon are two of the most commonly used elements in semiconductor production. The Gold-Silicon alloy (referred to chemically as “AuSi”) is a material of choice for fabrication of interconnects nearly for numerous MEMS devices. AuSi is often used, as well, to serve as a nanocatalyst in Solid-Liquid-Solid growth method to grow of silicon nanowire arrays. Additionally, AuSi eutectic alloys have been used in eutectic wafer bonding process in manufacturing and packaging of microchips and micro-electro mechanical systems (MEMS). In the role of a sealant, AuSi eutectic alloys are a prime source of outgassed materials.
The common use of Au and Si as a sealant in MEMS and semiconductor industry mentioned previously means that amorphous AuSi may be (unintentionally) formed in numerous manufacturing processes. Given its conductive properties, the deposition of the AuSi might well interfere or damage the MEMS device where the AuSi forms.
The word “eutectic” is also commonly applied to describe the composition that has the lowest possible melting point of an alloy. Eutectic (from Greek “Eutektos”, which means “easily melted”) implies a formation of a liquid alloy at the temperature that is lower than melting temperature of its components. For example, AuSi alloy with 19.5% atomic Si and 80.5% Au melts at T=363 C.°, while pure Au and pure Si are solid up to 1063 C.° and 1412 C.° respectively. Given the low temperature of formation, an ideal sealing method is selected with care must be taken to keep carriers below 363 C.° in order to minimize the unintended formation of AuSi.
One solution to the problem of compromised vacuum due to outgassing has been the insertion of a getter into the hermetically sealed electronic package to assure successful MEMS device operation for long duration applications. The term “getter” refers to materials, which chemically absorb active gases in a vacuum environment. The getter consists of a highly porous and mechanically stable packaging component installed inside the MEMS vacuum packaging chamber and subsequently activated by subjecting the sealed electronics package to a suitable combination of temperature and time. Activation of the getter, however, can also trigger the formation of eutectic alloys which, themselves, tend to outgas byproducts that compromise the vacuum within the electronics package.
What is needed is a sealing technology that prevents outgassing thereby eliminating the need for getters.